The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell power plant that minimizes free water within one or more fuel cells of the plant and eliminates free water within support systems of the plant so that the plant is freeze tolerant during shut down, start-up and steady-state operation in below freezing ambient temperatures.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing fluid and process oxidant reactant streams to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM. Use of such PEM fuel cell power plants to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing mechanical damage when the fuel cell generated water and/or any water coolant fluid freezes, and rapidly melting any frozen water during start-up after the fuel-cell powered vehicle has been shut down in sub-freezing conditions.
Accordingly there is a need for a fuel cell power plant that may be shut down in sub-freezing conditions, that does not sustain mechanical damage resulting from freezing, and that may be quickly started up without need to melt substantial quantities of water.
The invention is a fuel cell power plant having a reduced free water volume so that the one or more fuel cells and support systems of the plant are freeze tolerant during shut down, start-up, and steady-state operation of the plant in ambient temperatures below the freezing temperature of water.
The fuel cell power plant includes at least one fuel cell for generating electrical current from reducing fluid and process oxidant reactant streams; a coolant system, including a cooler plate secured in heat exchange relationship with the fuel cell that directs an antifreeze coolant through the cooler plate to remove heat from the fuel cell, a coolant accumulator in fluid communication with the cooler plate that stores the antifreeze coolant, and a coolant circulating line that directs the antifreeze coolant to flow from the coolant accumulator, through a coolant heat exchanger, through the cooler plate and back to the coolant accumulator; a fuel cell water collector in fluid communication between the fuel cell and the coolant accumulator that directs excess water removed from the fuel cell to the coolant accumulator or to a drain; a water vapor transfer system secured in fluid communication with the coolant accumulator that receives the antifreeze coolant from the accumulator and transfers water vapor out of the antifreeze; and, a fuel cell start-up system secured in fluid communication between the coolant accumulator and the fuel cell including a start-up heat exchanger that heats the antifreeze coolant, a start-up valve and a startup line secured between the start-up heat exchanger and the fuel cell for selectively directing the heated antifreeze coolant from the start-up heat exchanger through the cooler plate to heat the fuel cell. In an alternative embodiment, the fuel cell power plant also includes a fuel processing system secured in fluid communication with the water vapor transfer system and with the fuel cell that utilizes the water vapor in processing a hydrocarbon fuel for the fuel cell, wherein the start-up heat exchanger is secured in heat exchange relationship with the fuel processing system.
During operation and upon shut down of the fuel cell power plant, most of the excess water removed from the fuel cell is not cycled through the fuel cell as a coolant, but instead is directed through the fuel cell water collector into the coolant accumulator to mix with the antifreeze coolant, or from the water collector to be discharged to ambient through the drain. Therefore, the coolant system, water vapor transfer system, fuel cell start-up system, and fuel processing system all include the antifreeze coolant so that they have no free water that could freeze and damage the plant during operation and/or plant shut down in ambient conditions that are below the freezing temperature of water. Upon start-up of the plant, any free water within porous components of the fuel cell that might have to be thawed would be heated by the heated antifreeze coolant being directed from the start-up heat exchanger through the start-up valve and line and through the cooler plate. Additionally, any water vapor needed for the fuel processing components upon start-up of the plant in below freezing conditions would be transferred from the liquid antifreeze coolant within the water vapor transfer system into the fuel processing system. Consequently, the fuel cell power plant with a reduced free water volume of the present invention provides for start-up, steady-state operation, and plant shut down in below freezing ambient conditions with no free water within the coolant system, water vapor transfer system, start-up system, or fuel processing system of the plant. The water removed by the water collector may be vented to ambient, may be drained into the coolant accumulator, or may be supplied to a fuel processing system for processing a hydrocarbon fuel into the reducing fluid, depending upon system requirements and environmental considerations.
The antifreeze coolant is a low vapor pressure antifreeze having a partial pressure of the antifreeze above a solution of the antifreeze coolant and water at an operating temperature of the cell that is less than 0.005 mm Hg.
In a preferred embodiment, the fuel cell power plant may also include a burner that receives and combusts an anode exhaust stream passing out of the fuel cell to burn any unused fuel from the fuel cell, and the combusted anode exhaust stream is then directed to a direct mass and heat transfer device secured in mass transfer relationship with the process oxidant stream prior to the oxidant stream entering the fuel cell to transfer water and heat from the combusted anode exhaust stream into the process oxidant stream. In such an embodiment, the start-up heat exchanger may be secured in heat exchange relationship with the burner. In an additional embodiment, the burner may be positioned in heat exchange relationship with the water vapor transfer system to facilitate transfer of water vapor out of the antifreeze coolant. The fuel cell power plant may also be operated so that the antifreeze coolant is maintained at a pressure that is lower than a pressure of the reactant streams to facilitate containment of the antifreeze, and to reduce a possibility that the antifreeze coolant might poison catalysts of the cell. In an alternative embodiment, the fuel cell may include porous water transport plates which may also serve as reactant flow fields of the fuel cell. Water generated by the fuel cell may then be directed through the porous water transport plates into and through the fuel cell water collector to a drain, to the coolant accumulator, or to a fuel processing system. In a further embodiment, the fuel cell may not have any porous water transport plates, and instead, fuel cell water and any water within the reactant streams passes out of the fuel cell as water vapor and entrained water droplets within cathode and anode exhaust streams. In such an embodiment the exhaust streams are directed through the burner to combust un-combusted fuel, and the combusted plant exhaust stream is then directed through an air or water cooled water recovery condenser, and the condensed water is then directed into the coolant accumulator.
Accordingly, it is a general purpose of the present invention to provide a fuel cell power plant having a reduced free water volume that overcomes deficiencies of the prior art.
It is a more specific object to provide a fuel cell power plant having a reduced free water volume that minimizes free water within the fuel cell and reduces free water within support systems of the fuel cell power plant.
It is yet another object to provide a fuel cell power plant having a reduced free water volume during steady-state operation, shut down and start-up of the plant.
It is another object to provide a fuel cell power plant having a reduced free water volume that facilitates a rapid start-up of the power plant after the plant has been shut down in ambient conditions below the freezing temperature of water.
It is a further object to provide a fuel cell power plant having a reduced free water volume that prevents mechanical damage to the plant by freezing of free water during shut down of the power plant in ambient conditions below the freezing temperature of water.
It is yet another object to provide a fuel cell power plant having a reduced free water volume where the water is drained from the fuel cell by gravity.
These and other objects and advantages of the present fuel cell power plant having a reduced free water volume will become more readily apparent when the following description is read in conjunction with the accompanying drawings.